A fuel cell vehicle is driven by the electricity, which is generated by supplying hydrogen fuel stored in a hydrogen storage tank and oxygen in the atmosphere respectively to an anode and a cathode of a membrane-electrode assembly (MEA) of a fuel cell stack and inducing an electrochemical reaction including oxidation and reduction.
When used in an engine of the fuel cell vehicle, the production yield of the fuel cell stack may be required to increase. As such, the fuel cell stack may be produced by a process including the steps of: a step of attaching gas diffusion layers (GDL) to both sides of the membrane-electrode assembly by thermal pressing or using an adhesive; and alternately stacking and connecting several hundreds of these membrane-electrode assemblies with separating plates. The functions of each essential component necessary for constructing the fuel cell stack are as follows. The membrane-electrode assembly includes: the anode and the cathode each of which contains a carbon-supported platinum catalyst; and an electrolyte membrane made of a fluorine-sulfonic acid copolymer which produces electricity through electrochemical reactions such as oxidation and reduction reactions. The gas diffusion layer (GDL) may be a water-repellent double layer made of carbon fiber and carbon powder and supports the membrane-electrode assembly, controls the moving routes of gas and water, and transmits the electrons generated from the membrane-electrode assembly. The separating plate may be a waterproof plate made of a metal matrix provided with flow channels and the separating plate supports the membrane-electrode assembly and the gas diffusion layer, thereby providing the moving routes of reaction gases and water and transferring electrons from the gas diffusion layer to a collector plate.
Among the essential components necessary for constructing a fuel cell stack, the quality of the anode catalyst, the cathode catalyst and the polymer electrolyte membrane of the membrane-electrode assembly may influence substantially on the output and durability of the fuel cell stack. Particularly, the quality of the fuel cell stack may be reduced by structural defects such as pinholes, damages, splits of the polymer electrolyte membrane. Furthermore, the structural defects may further cause damage by fire, i.e. combustion reaction, due to the direct chemical reaction of oxygen and hydrogen, thereby causing a risk of a fire occurring in the fuel cell stack, rather than the electric power generation of the fuel cell stack by the electrochemical reaction of the hydrogen fuel and the oxygen oxidant.
Typically, in order to determine occurrence and state of defects of the electrolyte membrane in the membrane-electrode assembly, a gas permeation rate (unit: mL/min) of specific gas to the membrane-electrode assembly (MEA), or alternatively crossover value, was measured, and then the occurrence and state of defects thereof are determined based on the lower and upper limits of a reference value. In particularly, for the fuel cell membrane-electrode assembly, large pores (1.0×10−2˜5.0×102 μm) of the platinum (Pt) catalyst and the gas diffusion layer (GDL) and ultrafine pores (<1.0×10−3 μm) of the electrolyte membrane may simultaneously exist. As such, the inert gas which is not adsorbed on the wall of pores in the electrolyte membrane may move by viscous flow based on Poiseuille's law and by Knudsen flow based on free molecule flow according to pore size, such that the degree of discrimination of the occurrence and state of defects thereof is very low.
Further, the occurrence and state of defects of the electrolyte membrane may be determined by measuring open circuit voltage (OCV: current vs. voltage) which is an electrochemical measurement method or by measuring the difference in voltage under the predetermined current density. However, this method may not be appropriate such that micrometer-sized pinholes formed in the MEA having a microporous structure and predetermined gas permeability may be discriminated significantly.
Furthermore, the existence and reproducibility of pinholes in the unit cell prepared by thermally pressing of the electrolyte membrane and in the gas diffusion layer (GDL) may be typically observed by scanning electron microscope (SEM) or transmission electron microscope (TEM). However, although those have been used in a primary detection, such microscopic method may not be suitable for detecting the defects over the entire surface of the electrolyte membrane since the electronic microscopes may be limited to the local region or microregion of the electrolyte membrane. Further, the electrolyte membrane after this method may not be reused since the electron microscopic methods are destructive.
In the related arts, in addition to the above-mentioned methods of detecting defects using the gas permeation characteristics or electrochemical characteristic of the MEA, other methods of detecting defects using optical properties and electrochemical characteristics have also been used. However, defects over the entire region of the electrolyte membrane in the membrane-electrode assembly may not be detected since those methods may detect only the localized region of the membrane-electrode assembly or may not discriminate micropinholes, such that those methods may not be suitable as well.
It is to be understood that the foregoing description is provided to merely aid the understanding of the present invention, and does not mean that the present invention falls under the purview of the related art which was already known to those skilled in the art.